1. Field of the Invention
The present invention relates to a control device for controlling movement of a carriage of a printing head and, particularly, to a control device and method for controlling movement of a carriage of a printing head of a serial printer having a large inertia load and operable at high speed.
2. Description of the Prior Art
In a printer having a printing head carriage, a conventional carriage on which a printing head is fixedly mounted is moved by connecting the carriage to a motor through a power transmission mechanism comprising a wire or a timing belt and converting rotation of the motor into a linear movement of the carriage. Since, among printers, a printing head of a serial impact printer operable at high speed is heavy and in view of demand of reducing a size of the printer, a distance in which the carriage is accelerated or decelerated tends to be shortened. Therefore, vibration of the power transmission mechanism in accelerating or decelerating the carriage is propagated to the carriage and vibration of the carriage continues even after the carriage speed reaches an aimed value, resulting in degradation of printing quality.
In order to restrict such vibration and obtain a print of high quality, it has been usual to use a carriage movement control device having a follow-up control system constructed according to a direct detection method for directly detecting a moving speed of a carriage by means of a linear scale sensor (cf. Japanese Patent Application Laid-open No. Hei 1-234280) or an indirect detection method for indirectly detecting an amount of expansion or contraction of a power transmission mechanism by mounting speed sensors on pulleys on both a motor side and a driven side (Japanese Patent Application Laid-open No. Hei 1-238974).
However, the above-mentioned conventional scheme requires the expensive linear scale sensor or the expensive speed sensors on both the motor and follower sides and, further, the sensor or sensors must be mounted in place with high precision.
FIG. 1 is a block diagram showing another conventional scheme for restricting vibration and obtaining high print quality. In FIG. 1, a carriage driving mechanism 9 includes a carriage 92 on which a printing head 91 is mounted and which is connected to a power transmission mechanism 98 provided between a driving side pulley 96 and a driven side pulley 97 and an encoder 95 which detects rotation speed of a motor 94 fixedly secured to the driving side pulley 96 of the carriage driving mechanism 9. The encoder 95 may be constructed with a rotary encoder which generates 2 signals having phases different from each other by 90.degree.. The printing head 91 prints a printing sheet which is not shown on a platen 93 through an inked ribbon which is not shown.
A control portion 110 of the carriage movement control device shown in FIG. 1 drives the motor 94 by means of a phase-locked-loop (referred to as PLL hereinafter) functioning to synchronize phase of an output of a high stability reference oscillator 111 with phase of an output pulse signal y of the encoder 95 which corresponds to rotation speed of the motor 94. A phase comparator 112 of the carriage movement control device compares phase of the output pulse signal y of the encoder 95 with phase of the output of the reference oscillator 111 and generates a phase error signal 116. The phase error signal 116 phases through a low-pass filter 113 having an amplifier function and drives the motor 94 through a drive circuit 114. The motor 94 rotates correspondingly to a voltage of the phase error signal 116 and the encoder 95 generates a frequency signal corresponding to the rotation speed. Therefore, the motor 94 and the encoder 95 operates as a voltage controlled oscillator.
In case of PLL control, a variation of moving speed of a carriage in normal condition depends upon stability of an oscillator used. Therefore, such moving speed variation can be restricted to relatively small value if a highly stable oscillator is used. However, in order to accelerate the carriage bearing a heavy load from a stationary state thereof, a wide capture range W is required for PLL control. Capture range W is width between frequency at which the PLL control is not locked and frequency at which it is locked, and can be approximated by the following equation which is well known as G. S. Moschytz's approximation: EQU W=K.multidot..vertline.F(j.omega.).vertline.
where K is loop gain of PLL control, F(j.omega.) is transfer function of the low-pass filter, .omega. is angular velocity of the motor and j is imaginary number unit.
When the loop gain K is made large in order to increase the capture range W, the closed loop may become unstable and oscillate. In order to solve this problem, U.S. Pat. No. 4,457,639 issued to Nagai proposes a carriage movement control device. FIG. 2 is a block diagram showing the carriage movement control device of the Nagai patent. In FIG. 2 showing a closed loop circuit including a reference oscillator 111, a phase comparator 112, a low-pass filter 113 and a differentiating circuit 115. The capture range W is widened by improving gain in a high frequency range of the low-pass filter 113 by means of the differentiating circuit 115.
In the closed loop circuit shown in FIG. 2, however, since the low-pass filter 113 and the differentiating 115 are analog circuits, respectively, the capture range W depends upon accuracy of resistors and/or capacitors used in these circuits. Further, the differentiating circuit 115 which must compensate for a large inertial load is easily affected by noise. In addition thereto, if the reference oscillator 111 is stopped to operate for some reason, the phase comparator circuit 112 shall generate a very large phase error signal 116 with which a motor 94 driven thereby shall overrun. Further, since, in this example, a position information of a carriage 92, that is, rotation angle information of the motor, is not utilized, an exact stop position of the carriage 92 can not be determined when it is to be stopped and, in the worst case, the carriage 92 may collide with a structural frame of the device.